The invention is in the field of infrared intrusion detectors. Such detectors are designed to sense infrared radiation from persons or objects in a spatial region and to respond to movement by them. The detectors include one or more infrared sensors, with each sensor typically including two or more pyroelectric sensor elements for producing an electrical signal if incident infrared radiation varies. The infrared radiation enters a detector housing through an infrared-permeable entrance window and is focused onto the sensor elements by suitable optical elements, e.g., focusing mirrors or Fresnel-lens entrance windows.
For selective sensing of infrared radiation with wavelengths in a vicinity of 10 .mu.m as emitted by warm bodies, and as distinguished from extraneous electromagnetic radiation at other wavelengths, infrared intrusion detectors are provided with optical filters such as interference filters, for example. Such filters are preferably disposed near the pyrosensors.
It has been found that, even when equipped with high-quality interference filters, such infrared detectors respond to electromagnetic radiation at wavelengths considerably shorter than 10 .mu.m, causing false alarms. As a countermeasure, scatter filters have been included, taking the form of pigmented entrance windows with wavelength-dependent scattering of incident radiation. Such a pigmented entrance window is disclosed in European Patent Document EP-A-0 440 112, for example.
A similar effect can be achieved by a mirror surface which is roughened for desired wavelength selectivity. With such a surface, infrared radiation at predetermined wavelengths can be focused on a sensor element while extraneous radiation is diffusely scattered. Such a roughened mirror surface is disclosed in European Patent Document EP-A-0 617 389, for example.
With scatter filters and mirrors alike, the output signal of the pyrosensor depends on detector geometry, e.g., mirror geometry, pyrosensor aperture, and distance of the sensor from the scattering element. These parameters are then chosen to scatter the extraneous radiation in the detector so that it reaches the pyrosensor with an intensity below an alarm threshold. This tends to be difficult to achieve with sufficient certainty.